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Extreme Engineering

Martha Heckel

January 1, 1999

Extreme Engineering

Extreme: It's the buzzword of the '90s. Headaches are extreme. Pressure is extreme. Even sports are "Xtreme." But for Obika Nwobi extreme is no exaggeration: Nwobi, a doctoral candidate in aerospace engineering at Penn State, is studying how droplets of fuel and oxygen act under the extreme conditions of high-altitude pressure and temperature.

Under regular conditions, the droplets are spherical; they go from liquid to gas in predictable patterns before combustion takes place. When temperatures and pressures are extreme, however, the droplets lose their spherical shape. Their motion becomes less predictable. Extreme, or super-critical, conditions are produced by speed and atmospheric pressure—factors that more frequently affect airplanes than they do cars driving through the countryside. Yet knowing how molecules evaporate, disperse, and interact with one another inside a highly pressurized and superheated engine will provide a better understanding of combustion in general, Nwobi believes. And that knowledge could lead to improved engine designs, making propulsion smoother, faster, and safer, whether the vehicle is a NASA craft or a Chevy Nova.

The process of combustion has several steps. First, the liquid fuel evaporates. Then the fuel particles disperse to react with gaseous oxygen, creating multiple explosions inside the engine. These, in turn, produce energy to propel the craft. Combustion under extreme conditions generates more energy than is produced ordinarily. Researchers also know that, instead of remaining spherical, a fuel droplet becomes smooshed under these conditions, looking like a caved-in ping-pong ball.

This change in shape affects the molecular dynamics of the process—the force each molecule exerts on the others depending on its velocity, position, acceleration, and positive or negative charge—which affects how the fuel particles disperse and react. Using the principles of molecular dynamics, along with advanced computer technology, Nwobi is simulating the motion of these unspherical particles. "Assume we have a box of molecules, all in a computer, and we're letting them react, each with different potentials and different properties," he explains. "When atoms are far apart, they attract each other. As they get closer, they begin to repel each other." The bouncing becomes chaotic and difficult to predict. However, using recent advances in parallel supercomputing, Nwobi can take into account all of the molecule's so-called transport properties: shear viscosity (how fluids flow over surfaces), thermal conductivity (how heat travels through an element), and pressure (caused by both the atmosphere and friction). With as many as 16 computers working in parallel, he can track several thousand molecules, representing either oxygen, ethylene, or argon, as they move within his "box," or engine model."

Supercomputers save lots of time," says Nwobi. "In the '70s there wasn't enough power to do computations for engineers. Today, simple equations can be used to develop theories that couldn't be tested before. Then, single simulation could take months of reworking calculations to follow different paths and different atoms. Now, if many computers are working at once, it may take a day or two."

So, as hundreds of small dots run across his computer screen, Nwobi is learning more about evaporation, the effects of extreme conditions on droplets, and how combustion works. With a better basic understanding of these topics, he's helping other minds design a better engine—and take combustion to the extreme.

Obika Nwobi is a Ph.D. candidate in the department of aerospace engineering in the College of Engineering, 232 Hammond Bldg., University Park, PA 16802; 814-865-1965; oxn102@psu.edu. His adviser is Lyle N. Long, Ph.D., professor of aerospace engineering, 233 Hammond Bldg.; 865-1172; lnl@psu.edu. This project is funded by the Air Force Office for Scientific Research and by NASA.